Refining of crude oil to produce lubricating oil is one of the oldest refinery arts. Suitable crudes are fractionated to isolate a suitable boiling range material, usually in the 600.degree. to 1000.degree. F. range, to produce a distilled oil fraction. Various solvent purification steps are then used to reject components not suitable for lubricating stock.
Aromatics are too unstable, and refiners resort to various means to remove aromatics from potential lube fractions. While many solvents were proposed for aromatics extraction, furfural has been a preferred solvent since about 1933 when the first commercial furfural extraction units were built.
Furfural is denser than oil and related to formaldehyde. It is a solvent for aromatics. When furfural and a heavy oil fraction mix, the furfural dissolves much of the aromatics content of the heavy oil. Upon settling, an extract phase or dense furfural phase containing most of the aromatics separates from a raffinate phase of lighter hydrocarbons with a reduced amount of aromatics. As in most liquid/liquid extraction processes the separation is not perfect. Some aromatics remain in the raffinate and some furfural dissolves in the raffinate. Fractionation of the extract and raffinate recovers the furfural solvent for reuse.
Some representative patents on preparation of lubricants by solvent extraction include U.S. Pat. Nos. 2,698,276, 3,488,283 and 4,208,263 which are incorporated by reference.
Although in use for more than 60 years, solvent recovery is a problem, one which has become a severe problem as costs of energy have soared.
At its inception, furfural recovery was straightforward, and wasted energy. Furfural was believed to be thermally stable, and efforts were directed to its recovery rather than its preservation. Recovery by simple multiple stage heating and flashing or evaporation was straightforward.
Thermal stability of furfural was a "fact" since the 40's. Dunlop and Peters, Jr. reported in Ind. & Eng. Chemistry V. 32 #12, 12/40:
Thermal Stability of Furfural: The data presented show that, under the specific conditions of these experiments, refined furfural is quite stable . . . from an industrial standpoint furfural is thermally stable. No commercial process is known wherein furfural is subjected to temperatures of the magnitude of 230.degree.-275.degree. C. for more than a few minutes, and . . . it is a matter of hours before a change in the properties of furfural can be detected." (Abstract)
Several stages of furfural recovery were typically used, each at successively lower pressures to improve vaporization. Typically three towers were used, each with a heater, and each with an overhead vapor condenser. This did a fine job at recovering furfural, but required vaporization and condensation of large amounts of material several times. Such energy profligacy might be acceptable when energy was cheap but not when it was dear. It could not be tolerated in furfural aromatic extraction units where from 10 to 50 moles of solvent are present per mole of extract.
In response, engineers modified the furfural recovery section. Typically three stages of furfural recovery are still used, but each stage operates at higher, rather than lower, pressure. Condensing vapor from downstream stages supplies much or all of the heat input needed by each upstream stage. In many units the only heat input is to the last, typically third stage, furfural recovery tower. Liquid would be pumped from the first stage to a second stage operating at a higher pressure, and pumped from the second stage to the third stage. Pumping was necessary so that the vapors released in a downstream stage would have a pressure high enough that condensing vapors would heat an upstream stage.
In many units this pressure reversal also reversed the rates of furfural recovery. In most prior art furfural recovery processes, with decreasing pressure in each stage, most of the furfural was recovered in the first stage, and lesser amounts in succeeding stages. A steam stripper could then be used as a final clean up device, to create a pseudo vacuum and remove the last traces of furfural from the extract.
In the modern, ascending pressure approach more furfural is vaporized in downstream units to provide enough condensing vapor to heat the upstream units. Typically, the first stage flash, the low pressure flash (LP Flash) operates at 5-20 psig while the second stage, the medium pressure flash (MP Flash), is at 10-40 psig. The third stage of recovery, a high pressure flash (HP Flash) may operate at 15-50 psig. A typical unit operates with pressures of about 10-20-35 psig in stages 1-2-3, respectively.
Large amounts of furfural usually remain despite three stages of flashing because of the higher pressures now used in the downstream stages. The stripper, typically operating under a vacuum, must recover fairly large amounts of furfural in a modern ascending pressure unit. For a unit recovering 2600 pound moles per hour of furfural from 60 moles/hr of extract, furfural and extract traffic is:
______________________________________ PSIG moles/hr furfural moles/hr extract ______________________________________ LP Flash 10 600 60 MP Flash 20 800 60 HP Flash 35 1000 60 Stripper -10 200 60 ______________________________________
The table above reports the amount of furfural vaporized at each stage, and the amount of extract passing through each stage as a liquid. The LP Flash vaporizes 600 lb moles/hour of furfural, while the HP Flash has to vaporize significantly more (1000 lb moles/hour) furfural. The stripper is required to remove a lot of furfural under vacuum conditions.
Many commercial units are about this size, but the numbers have been rounded to one significant figure to illustrate simply the way modern units operate. The feed to the furfural recovery section has almost 40 times as much furfural as extract, on a molar basis.
Furfural vaporization decreases as pressure increases, so ever higher temperatures are needed to remove increasing amounts of furfural at the higher pressures of each downstream flash stage. Much of the furfural is recovered at the highest pressure, putting severe demands on the fired heater associated with the HP flash. Quite a lot of furfural remains in the liquid from the high pressure flash, so a stripper, or a vacuum flash and a stripper, typically recover additional amounts of furfural from the high pressure flash liquid fraction.
Some refineries use only two stages of flashing, rather than the three stages described above, and some may use four or more stages. The common thread in all of these modern furfural recovery units has been use of higher pressure and higher temperature in downstream units to generate furfural vapor which can be used to heat at least one upstream flash unit.
This modern, ascending pressure approach saves energy and has been widely adopted in the refinery industry. Instead of expending the energy to vaporize 2400 moles of furfural (the total amount removed in three stages of flashing), a refiner need only vaporize the 1000 moles vaporized in the third stage, or high pressure flash. These vapors are at a high enough pressure that they will condense, and release their heat, in heat exchange with feed to the MP Flash. The vapor from the MP flash in turn heats the feed to the LP flash. The refiner vaporizes 2600 moles of furfural, but expends only enough energy to vaporize 1000 moles, because of the multiple evaporation effect.
There is little capital or operating cost involved in pumping liquids from tower #1 operating at, e.g., 10 psig into tower #2 operating at 20 psig. This new approach has been widely adopted. It solves an energy problem, but creates a furfural degradation problem.
At the pressure in the high pressure flash, typically 35-50 psig, the temperature required for adequate furfural evaporation rates is usually above 450.degree. F. Refiners now use fired heaters, many of which operate with tube wall temperatures over 500.degree. F. Such high temperatures are needed to evaporate furfural from the extract oil fraction at the pressures required in the high pressure flash. Such temperatures also are high enough to degrade the furfural.
Furfural is reactive at these temperatures and decomposes into a hard crusty coke which deposits in refinery processing equipment. These coke deposits build up until the furfural extraction unit must be shut down for cleaning. A commercial unit is typically cleaned every year, more frequently in some cases. The unit may be down for one to two weeks, causing lost production time. Thermal decomposition of furfural also means a loss of valuable solvent, and potential contamination of the extract oil product fractions.
Furfural operators knew of the problem, but most still shut their units down once a year or so for cleaning. Some resort to adding amines or proprietary treatments from vendors such as Aquachem to reduce the rate of decomposition of furfural, or to keep the decomposition products in suspension. Chemical treatments help, but are expensive.
Some refiners extend run lengths by downgrading their equipment capacity. Thus a fired heater may not be used to capacity, or is deliberately oversized, so that tube or wall temperatures (and furfural degradation) may be reduced.
I wanted to reduce or end unit shutdowns, and also reduce or end furfural decomposition. I discovered a multi-step way to recover furfural solvent from extract oil, which used most of the existing equipment in a lube refinery for at least the initial stage(s) of furfural recovery and a new type of furfural recovery, for some or all of the final stage(s). The solution was to use a thin film evaporator, such as a long-tube vertical evaporator or agitated thin-film evaporator, to do some or all of the final stages of furfural recovery.
None of these evaporators are new, they have been available to refiners for decades. More details about Long-tube vertical (LTV) evaporators, and the falling-film version of the LTV may be taken from Perry's Chemical Engineer's Handbook, 6th Edition, in the section on EVAPORATORS from 11-31 to 11-41, incorporated by reference.
The equipment is also described in Kirk-Othmer, ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, 2nd edition, Volume 8, page 568, and FIG. 7, which are incorporated by reference. As stated therein, the long-tube vertical evaporator consists simply of a vertical single-pass shell-and-tube heat exchanger surmounted by a V/L separator. Tubes are typically 2" by 24-36' in black-liquor service. This size is reported to be the cheapest form in which heating surface can be provided in any evaporator.
Another description of the equipment is provided by McKetta and Cunningham in Encyclopedia of Chemical Processing and Design, Volume 20, Evaporators and Evaporation, starting at page 415, and especially FIG. 4, which is incorporated by reference. They reported the tubes are usually about 50 mm diameter by 7 to 10 m long, packaging a large amount of heating surface in a single shippable tube bundle. This reference reports that long tube vertical evaporators are the most common of the film type evaporators, with more evaporative capacity than all other steam-heated types combined.
Such equipment is available commercially from vendors such as Blaw Knox and Swenson.
Such evaporators have never been used for furfural recovery in a lube refinery. This may be due in part to difficulties which would be experienced if such devices were used as the sole means of furfural recovery. As stated in Kirk Othmer, Volume 9, Evaporation, in the section on wiped-film evaporators: "Such evaporators exhibit poor heat transfer performance on low viscosity fluids because of the added resistance of the metal wall." Another concern is that these devices are expensive, and too costly to use for recovery of furfural solvent, in view of the large molar ratios of furfural to extract which are involved.
I realized a hybrid approach was needed. Most of the furfural recovery should be conventional, using flash separation. Preferably thin film evaporation is used only for the final stages of furfural recovery, when the temperatures are highest. In this way conventional, and usually existing, equipment can be used for economical recovery of most of the furfural for no capital and low operating expense, while the high capital cost solvent recovery step is reserved for the final stages of the operation. My process will involve some capital cost for LTV or wiped film evaporators, but the energy required to recover furfural in this type of evaporator will be less than a fired heater. Thus my process can save energy and eliminate a fired heater, in some installations. In any installation rapid payouts for new equipment will occur. The modest capital costs associated with the selective use of thin film evaporators can be recovered in just a few months in many instances, if proper credit is taken for all the costs of furfural degradation in fired heaters.